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First astatine-labeled compound shipped in the U.S.
The Department of Energy’s National Isotope Development Center (NIDC) on March 31 announced the successful long-distance shipment in the United States of a biologically active compound labeled with the medical radioisotope astatine-211 (At-211). Because previous shipments have included only the “bare” isotope, the NIDC has described the development as “unleashing medical innovation.”
Coleridge A. Wilkins
Nuclear Science and Engineering | Volume 14 | Number 4 | December 1962 | Pages 346-357
Technical Paper | doi.org/10.13182/NSE62-A26241
Articles are hosted by Taylor and Francis Online.
This paper involves the application of some modern probability methods to the problem of the slowing-down of neutrons, and attempts to provide a framework both for the exposition and computation of various phenomena in this field. It is shown that under certain conditions, the slowing-down of neutrons is a renewal process and renewal theoretical results apply. For instance, if scattering is isotropic and all cross sections similarly varying, then (in lethargy) slowing-down is certainly a renewal process. After investigating some aspects of moderation in hydrogen, and some incidental extensions of results for hydrogen, it is shown that for renewal types of slowing-down from a monoenergetic source, the Laplace transform of the rth moment of the number of collisions at lethargies below u may be obtained by differentiating (with respect to t) a generating function of the form This form applies to some types of anisotropic scattering as well as to isotropic scattering. The expressions derived are then extended to a distributed source by a method from reactor theory, and some resulting expressions are checked against corresponding renewal formulas. The asymptotic distribution of the number of collisions to slow down is found from renewal theory, from which it is shown that when scattering is isotropic, the spread of the number of collisions to thermalize in a light moderator is relatively greater than in a heavy moderator, the coefficient of variation being proportional to for a light species and for a heavy one. The asymptotic form of the density of nth collisions is investigated, and the form derived is compared with another ascribed to Dancoff. The preceding theory is then applied to a particular case of anisotropic scattering which occurs above about 100 kev. Finally, an exact expression is obtained (for similarly varying cross sections and zero absorption) for the probability that, in a mixture of n species, a neutron has r collisions at lethargies below u, precisely k of which are with a given species. The given species is then taken to be very heavy and the exact expression approximated accordingly.
